[AeroModder]

My advice would be to work on the underside. A good undertray will give you FAR more benefits than a fastback mod would, especially with your already smooth rear window.

One way you can improve topside aero is remove the spoiler and install the fin-shaped radio antenna you see on BMWs and such, which doubles as a vortex generator. That spoiler's design will give rear downforce unlike the strictly cosmetic ones, so removing it will make the rear end feel loose on the freeway. You'll need to weigh the pros and cons on that for yourself.

Also, seeing how aftermarket Mustang parts are so common, go with a set of lowering springs.